368 Mr. A. A. Common [Feb. 26, 



that would be just beyond the power of the eye, however long the 

 gazing was continued, could be photographed with a sufficiently long 

 exposure ; and this holds good whatever optical power be employed 

 to increase the amount of light brought to the eye ; as with the same 

 optical power, the power of the sensitive plate, if allowed sufficient 

 exposure, will always be greater. There is from this another conse- 

 quen^e: — The enlargement of an image produced by a telescope is 

 limited in one direction by the light, faint objects becoming too faint 

 to be seen if greatly enlarged, the sensitive plate, however, may still 

 utilise this image. There are other points of difference between the 

 action of the retina and of the sensitive plate. The power of the 

 latter is greater to record rays of light of quicker vibration, and it 

 may be possible to obtain photographs of celestial objects radiating 

 light that the eye is not adapted to receive ; and it is also quite 

 possible that plates may be prepared that will be sensitive to the 

 visual rays so that the magnitudes of stars in stellar photographs 

 would agree with magnitudes as obtained by the eye. 



Though the image seems to be clearly formed in the eye over a 

 large angular extent, the central parts only are clearly seen, and the 

 image has to be traversed across this central part piece by piece to be 

 IH'operly examined. 



With the sensitive plate, the image, no matter how complex, acts 

 equally over its extent and records itself with fidelity. 



As was well said many years ago by Dr. de la Eue, who has been 

 rightly called the father of astronomical j)hotography, " the sensitive 

 film is a retina that never forgets." 



I will try and show to-night how important the difference between 

 the ordinary eye observations and the work done by photography 

 may become ; not only in cases where the ordinary visual observa- 

 tions have been used, but in cases where the use of the micrometer 

 attached to the telescope has been the only means of accurate observa- 

 tions. Looking for a moment at the history of our subject, it appears 

 that the earliest application of photography to a celestial object was 

 made by Professor J. W. Draper in 1840 within one year of the 

 announcement by Daguerre of the details of the photographic j^rocess 

 with which his name will always be associated. Professor Draper 

 obtained a picture of the moon in twenty minutes, using a lens and a 

 heliostat. 



An extract from the minutes of the New York Lyceum of Natural 

 History was to this effect : — 



" March 23, 1840. — Dr. Draper announced that he had succeeded 

 in getting a representation of the moon's surface by the daguerreo- 

 ty})e. The time occupied was twenty minutes, and the size of the 

 figure about one inch in diameter. Daguerre had attempted the same 

 thiug but did not succeed. This is the first time tliat anything like 

 a distinct representation of the moon's surface has been obtained. — 

 Signed, Kobert U. Brownne, Secretary." 



Kememberiug that this entry was made less than one year after 



